lol at anyone who said Cruz. Compared to pro boxing, Cruz's "boxing" is fucking terrible. Awful footwork too, he just randomly shuffles around and throws obvious, telegraphed punches. He is a very good all round MMA fighter, but a boxer he is not. Your average solid journeyman would school Cruz with no trouble at all.
The two most obvious choices are Cody and Stipe, they both have a solid amateur background, they know how to box to some degree. They can both generate power punches like a boxer, ie effortlessly, with short shots, from different positions, and without much windup.
Cody would be my pick out of anyone in MMA, to potentially get the best win % in boxing. With a good boxing trainer, get him to be a bit more clinical and compact. He'd obviously get absolutely schooled by any elite boxer, but unlike Conor, his power and cardio would translate to boxing, enough to have some degree of success anyway.